This invention relates generally to the production of higher aliphatic, olefins from the oligomerization of lighter aliphatic olefins.
Prior Art
Processes for the oligomerization of lighter olefins to produce C6 and higher carbon number olefins are well known. Oligomerization processes can be used to produce plasticizer components from propylene. Additionally, oligomerization processes have been long employed to produce good quality motor fuel from butylene. Such oligomerization processes are also referred to as catalytic condensation and polymerization with the resulting motor fuel often referred to as polymer gasoline. Methods have always been sought to improve the octane number of the gasoline boiling range oligomerization products. In addition, the oligomerization process is also susceptible to catalyst fouling from the condensation of heavy oligomers into coke that covers the catalyst.
Another process that has met the continuing demand for the conversion of light hydrocarbons into high octane motor fuels was the alkylation of isobutane with propylene, butenes and amylenes using a hydrofluoric acid (HF) catalyst, commonly referred to as HF alkylation. The HF process has provided a highly successful method for the production of high octane motor fuels.
A number of arrangements are known for using oligomerization in combination with other processes such as saturation and dehydrogenation as substitutes for acid catalyzed isomerization alkylation. Patents disclosing the dehydrogenation of light paraffin stream with oligomerization of the dehydrogenation effluent include, U.S. Pat. No. 4,393,259, U.S. Pat. No. 5,049,360, U.S. Pat. No. 4,749,820, U.S. Pat. No. 4,304,948 and U.S. Pat. No. 2,526,966.
In the oligomerization method of the indirect alkylation process set forth in, for example, U.S. Pat. No. 5,990,367, lighter aliphatic olefins such as C3 or C4 are contacted with a solid phosphoric acid catalyst in the presence of a higher paraffin diluent such as cyclohexane or octane. The presence of the paraffin diluent is believed to promote the oligomerization in the liquid phase to yield predominantly dimerized butylene or trimerized propylene oligomers such as C8 and C9 olefins. The higher aliphatic olefins can be saturated to provide fuel or plasticizer components.
It is highly desirable to operate the oligomerization reaction under plug flow conditions to assure uniform conversion along the reaction front. Maintenance of plug flow conditions assures a tighter product distribution. Without plug flow conditions, channeling and even recirculation can result. In xe2x80x9cchannelingxe2x80x9d, segments of the reaction front move downwardly more quickly than other segments of the reaction front causing bypassing of downstream product fluid by the upstream reactor fluid. This flow instability is also called xe2x80x9cfingeringxe2x80x9d and is a result of the fluid wanting to achieve a lower energy state. xe2x80x9cRecirculationxe2x80x9d involves swirling of the reactants against the direction of flow. Channeling can cause underconversion and overconversion of reactants to product; whereas, recirculation can have the same effect but to a greater degree. Overconversion can generate even greater temperatures than desired for the oligomerization reaction to proceed and can cause the catalyst to degrade by deposition of carbon particles on the catalyst which is a phenomenon known as xe2x80x9ccokingxe2x80x9d. These effects operate to spread the product distribution away from desired products, thereby diminishing product value and consistency.
It was originally thought that a downflow reactor scheme would provide sufficient reaction front stability to operate under plug flow conditions. Pilot plant studies did not alert to the fact that plug flow could not be maintained under downflow oligomerization conditions. Modeling was conducted to study the stability, of the reaction front under oligomerization conditions. The study revealed not only that downflow aliphatic oligomerization would be unstable, but that it would be far less stable than anticipated. Surprisingly, the modeling study revealed that downflow, was so unstable that channeling and even recirculation of reactants could take place under certain conditions.
The density of the liquid mixture in the aliphatic oligomerization reaction decreases proportionally with the progress of the oligomerization. The relatively high heat of reaction from oligomerization generates very high temperatures causing the reaction products to be less dense and more buoyant relative to the reactants even though the higher aliphatic olefin products are more dense than the lower aliphatic olefin reactants at equivalent conditions. The higher temperature effects a greater reduction in density than the composition change increases the density of the products. The viscosity of the liquid mixture in the oligomerization also decreases proportionally with progress of the oligomerization, but the effect of viscosity on stability is much less prominent than is the effect of density. Flow instability occurs when the denser inlet fluid bypasses the less dense product fluid during operation in downflow.
Upflow reactors with and without fixed catalyst beds are disclosed in the art. U.S. Pat. No. 5,789,640 discloses an upflow fluidized bed system using solid acid catalysts. U.S. Pat. No. 4,255,352 discloses upflow through a series of tank reactors to react an olefinic hydrocarbon and an olefinically unsaturated nitrile in the presence of a diluent predominantly comprising water to produce unsaturated dinitriles. The latter patent discloses the use of promoters which it defines to include catalysts without discussion of fixing the catalyst bed. U.S. Pat. No. 6,013,845 discloses producing bisphenol from dimethyl ketone and phenol in a fluidized catalyst bed. Backmixing of catalyst and the reactor feed is minimized by packing the bed with randomly oriented packing.
Both U.S. Pat. No. 3,560,167 and U.S. Pat. No. 4,801,432 disclose upflow reactors with fixed catalyst beds. Both reactors are equipped for at least one gaseous reactant, although the reactions take place partially in the liquid phase, and mechanical hold-down structures are required to maintain the stability of the catalyst bed.
U.S. Pat. No. 4,695,665, U.S. Pat. No. 4,051,191 and U.S. Pat. No. 4,343,957 disclose upflow processes for the production of cumene using solid phosphoric acid in fixed catalyst beds. The advisability of using an upflow scheme for an oligomerization reaction of aliphatic olefins to obtain plug flow conditions is not disclosed, nor is there any indication of the extent of the instability of an aliphatic oligomerization reaction proceeding in downflow mode.
It is an object of this invention to improve the plug flow stability and product distribution of an aliphatic olefin oligomerization reaction by operating the reaction in an upflow mode.
It has been surprisingly found that operating an oligomerization of lighter aliphatic olefins in the presence of a solid acidic catalyst and heavy paraffins in an upflow mode enables maintenance of plug flow conditions far better than operation of the oligomerization in the downflow mode. It was not even understood until modeling experimentation was undertaken how poorly the oligomerization of lighter aliphatic olefins would proceed in downflow mode.
Accordingly, an embodiment of the present invention comprises an oligomerization process for the production of higher aliphatic olefins. The process comprises passing a liquid oligomerization feed stream comprising lighter aliphatic olefins to a reactor vessel. The liquid oligomerization feed stream is transported upwardly in the reactor vessel against gravity through a fixed bed of solid oligomerization catalyst under oligomerization conditions. The catalyst has a Hammett acidity value of xe2x88x924 or less. A liquid saturate stream comprising paraffins is passed into contact with the feed stream and the catalyst. A liquid oligomerization effluent stream comprising paraffins and product higher aliphatic olefins is then recovered.
In another embodiment of the present invention, the feed stream comprises C3 or higher aliphatic olefins, the catalyst is a solid phosphoric acid catalyst, the liquid saturate stream comprises C5 or higher paraffins and the liquid oligomerization effluent stream comprises C6 or higher aliphatic olefin product.
In a further embodiment of the present invention, the oligomerization feed stream has a first density and the oligomerization effluent stream has a second density that is less than the first density of the oligomerization feed stream.
In still further embodiments of the invention, the oligomerization conditions include a temperature of 93xc2x0 to 260xc2x0 C. (200xc2x0 to 500xc2x0 F.), a pressure of 690 to 10342 kPa (100 to 1500 psig) and a liquid hourly space velocity of 0.5 to 5 hrxe2x88x921. Preferably, the oligomerization conditions include a temperature in the range of 149xc2x0 to 232xc2x0 C. (300xc2x0 to 450xc2x0 F.).
In even further embodiments of the invention, the oligomerization effluent stream is passed to a separator and separated into a product stream comprising higher aliphatic olefins and paraffins or at least a portion of the paraffins is recycled to the reactor vessel. Additionally, in an embodiment, at least a portion of the saturate stream enters the reactor vessel with the feed stream.
In other embodiments of the invention, it is contemplated that oligomerization will occur predominantly in the liquid phase, that the reactor vessel will include more than one fixed catalyst bed or that an inert material is disposed in the reactor vessel between a fixed bed of catalyst and a reactor feed inlet.
Moreover, other additional embodiments of the invention include that the lighter aliphatic olefins include butenes, the paraffins in the saturate stream have a carbon number of at least 6 or the product higher aliphatic olefins include octenes. Additionally, an embodiment of the invention contemplates that the product higher aliphatic olefins comprise dimerized or trimerized lighter aliphatic olefins.
Other objects, embodiments and details of this invention will be provided in the following detailed disclosure of the invention.